More than 3,900 flights were canceled and another 6,900 were delayed in the U.S. on Monday as frigid temperatures, snow and ice gripped states throughout the Midwest and along the East Coast.

Temperatures fell below zero across the Midwest and into the teens in some southern states, thanks to the so-called polar vortex pushing Arctic air farther south. In northern Minnesota, the temperature dropped to as low as –40°F with wind chills hovering around –50°F. The Arctic weather led some cities to keep students home from school.

The New York Timesreports that travelers faced a double-whammy Monday, as federal regulations requiring airline pilots to rest for longer periods between flights took effect just as winter weather wrecked havoc with flights.

A cornucopia of winter-weather systems have struck pockets of the U.S. over the past week, with snow, ice and fierce winds battering cities since the New Year began. According to flight-tracking website FlightAware.com, more than 17,000 flights have been delayed in the U.S. over the past week, while nearly 9,000 were canceled. Winter Storm Hercules, which blanketed the Midwest and Northeast last Friday, is said to have caused 16 deaths.

Unfortunately for residents across an estimated 1.5 million sq. mi. of the U.S., the National Weather Service reports that temperatures are expected to fall in more states on Tuesday. Half of the lower 48 states are expected to have wind chills below zero.